Method for evaluating the adsorption/desorption equilibrium of methane in coal and its impact on desorption hysteresis loops

The isothermal adsorption of methane in coal is essential for understanding the adsorption mechanism. The accuracy of equilibrium adsorption measurements directly impacts the reliability and scientific validity of the experimental results. This study conducted high-pressure methane isothermal adsorption/desorption experiments on coal samples with varying pore structures to investigate the impact of the adsorption equilibrium evaluation method on the results. The study found that using the fixed-time method to determine the equilibrium adsorption volume can lead to significant errors, particularly for samples with complex fluctuations in the adsorption/desorption curves. Based on the observed behavior of the adsorption curve, the slope method was proposed to determine the equilibrium adsorption volume. This method first smooths the raw data to extract the overall trend and then determines the equilibrium adsorption volume based on changes in the slope. Comparing the results of the two methods revealed that the slope method effectively addressed anomalies in the adsorption/desorption curves and significantly improved the fitting of the Langmuir model. For instance, the goodness of fit for the Shenmu Mine column sample increased from 0.7803 to 0.9960. Additionally, hysteresis analysis revealed that the hysteresis index of the slope method was significantly lower than that of the fixed-time method, indicating that the accuracy of the experimental results plays a critical role in desorption hysteresis.